Advances in the floral structural characterization of the major subclades of Malpighiales, one of the largest orders of flowering plants

Background and Aims Malpighiales are one of the largest angiosperm orders and have undergone radical systematic restructuring based on molecular phylogenetic studies. The clade has been recalcitrant to molecular phylogenetic reconstruction, but has become much more resolved at the suprafamilial level. It now contains so many newly identified clades that there is an urgent need for comparative studies to understand their structure, biology and evolution. This is especially true because the order contains a disproportionally large diversity of rain forest species and includes numerous agriculturally important plants. This study is a first broad systematic step in this endeavour. It focuses on a comparative structural overview of the flowers across all recently identified suprafamilial clades of Malpighiales, and points towards areas that desperately need attention. Methods The phylogenetic comparative analysis of floral structure for the order is based on our previously published studies on four suprafamilial clades of Malpighiales, including also four related rosid orders (Celastrales, Crossosomatales, Cucurbitales, Oxalidales). In addition, the results are compiled from a survey of over 3000 publications on macrosystematics, floral structure and embryology across all orders of the core eudicots. Key Results Most new suprafamilial clades within Malpighiales are well supported by floral structural features. Inner morphological structures of the gynoecium (i.e. stigmatic lobes, inner shape of the locules, placentation, presence of obturators) and ovules (i.e. structure of the nucellus, thickness of the integuments, presence of vascular bundles in the integuments, presence of an endothelium in the inner integument) appear to be especially suitable for characterizing suprafamilial clades within Malpighiales. Conclusions Although the current phylogenetic reconstruction of Malpighiales is much improved compared with earlier versions, it is incomplete, and further focused phylogenetic and morphological studies are needed. Once all major subclades of Malpighiales are elucidated, more in-depth studies on promising structural features can be conducted. In addition, once the phylogenetic tree of Malpighiales, including closely related orders, is more fully resolved, character optimization studies will be possible to reconstruct evolution of structural and biological features within the orde

A Comparison of Floral Structures of Anisophylleaceae and Cunoniaceae and the Problem of their Systematic Position

Flowers of Anisophyllea(Anisophylleaceae, Cucurbitales) andCeratopetalum (Cunoniaceae, Oxalidales) are surprisingly similar in appearance. To date, these families have never been interpreted as closely related, and even in present molecular (rbcL) studies they appear in different orders of eurosids I (APG, Annals of the Missouri Botanical Garden85:531-553, 1998). In this investigation, flowers of selected taxa of both families are morphologically and anatomically compared. In addition, previous work on the two families is reviewed. The results strongly emphasize the great similarity in all floral organs. Some special similarities include the occurrence of trimerous flowers, isomerous organ whorls (including the gynoecium), valvate sepals, digitate petals, obdiplostemony, incurved filaments in bud with similar anthers, similar pollen, similar nectaries, carpels with free styles, a canal in the centre of each individual carpel as well as in the centre of the entire gynoecium along the symplicate zone, and similar ovules with a slit-shaped micropyle. In addition, recently recovered Late Cretaceous floral fossils that share features of both families further emphasize a potential close relationship. However, if more extensive molecular studies are performed in the future that support the current disparate position of the two families, then an explanation of the biological/functional similarities in floral structure should be attempted: specifically, whether this suite of features is a symplesiomorphy for basal rosids, or an autapomorphy for each family. Copyright 2001 Annals of Botany Compan

Cunoniaceae in the Cretaceous of Europe: Evidence from Fossil Flowers

Fossil flowers of the Cunoniaceae from Late Cretaceous sediments of southern Sweden are described in detail. The flowers are small, bisexual, actinomorphic, tetramerous with broadly attached valvate sepals; they have narrowly attached petals; eight stamens in two whorls; a massive, lobed nectary; a semi-inferior, syncarpous gynoecium with axile placentation; numerous ovules; separate styles; and peltate, probably secretory, trichomes. They share many features with extant representatives of both the Cunoniaceae and Anisophylleaceae. However, the gynoecium structure in particular indicates a closer relationship to the Cunoniaceae. The floral characters are not specific for any extant genus of the family and therefore a new genus and species, Platydiscus peltatus gen. et sp. nov., is formally described. This is the first record of cunoniaceous floral structures from the Northern Hemisphere and the oldest record of Cunoniaceae flowers worldwide. Copyright 2001 Annals of Botany Compan

Comparative floral structure and systematics of the clade of Lophopyxidaceae and Putranjivaceae (Malpighiales)

In molecular phylogenetic studies, Lophopyxidaceae and Putranjivaceae are well supported as sisters in the large rosid order Malpighiales. As the floral structure of both families is poorly known and the two families have never been compared, the present comparative study was carried out, as part of a larger project on the comparative floral structure of Malpighiales, using microtome section series and scanning electron microscopy (SEM) studies. Similar to other angiosperm clades, it appears that the structure of the ovules is a strong marker for suprafamilial relationships in Malpighiales. Both families have two collateral pendant antitropous ovules per carpel associated with obturators (as in some Euphorbiaceae s.l., to which Putranjivaceae belonged in earlier classifications). However, in contrast with Euphorbiaceae s.l., the ovules are not crassinucellar, but either incompletely tenuinucellar or only weakly crassinucellar with a long and conspicuously slender nucellus and an endothelium, and do not have a nucellar beak, but a normal micropyle, features they share with families other than Euphorbiaceae s.l. among Malpighiales. Other shared features of the two families include the following. The outer sepals tend to be smaller than the inner ones and the sepals do not protect the gynoecium in older buds. Sepals of some taxa have a single vascular trace. A short zone of synsepaly tends to be present. Stamens tend to be antesepalous in haplostemonous flowers. A short gynophore is present. The synascidiate zone extends up to above the placenta, but is restricted to the ovary in taxa with more than one carpel. The micropyle is formed by the inner integument. The ventral carpel slits extend down into the synascidiate zone as postgenitally fused furrows. The carpels have a broad dorsal band of vascular bundles in the style. The overall floral structure of the two families corroborates their sister position well and does not support the earlier association of Putranjivaceae with Euphorbiaceae s.l. or of Lophopyxidaceae with Geraniales–Sapindales–Celastrales, which rely on shared superficial floral similarities

Comparative floral structure and systematics in Rhizophoraceae, Erythroxylaceae and the potentially related Ctenolophonaceae, Linaceae, Irvingiaceae and Caryocaraceae (Malpighiales)

Within the rosid order Malpighiales, Rhizophoraceae and Erythroxylaceae (1) are strongly supported as sisters in molecular phylogenetic studies and possibly form a clade with either Ctenolophonaceae (2) or with Linaceae, Irvingiaceae and Caryocaraceae (less well supported) (3). In order to assess the validity of these relationships from a floral structural point of view, these families are comparatively studied for the first time in terms of their floral morphology, anatomy and histology. Overall floral structure reflects the molecular results quite well and Rhizophoraceae and Erythroxylaceae are well supported as closely related. Ctenolophonaceae share some unusual floral features (potential synapomorphies) with Rhizophoraceae and Erythroxylaceae. In contrast, Linaceae, Irvingiaceae and Caryocaraceae are not clearly supported as a clade, or as closely related to Rhizophoraceae and Erythroxylaceae, as their shared features are probably mainly symplesiomorphies at the level of Malpighiales or a (still undefined) larger subclade of Malpighales, rather than synapomorphies. Rhizophoraceae and Erythroxylaceae share (among other features) conduplicate petals enwrapping stamens in bud, antepetalous stamens longer than antesepalous ones, a nectariferous androecial tube with attachment of the two stamen whorls at different positions: one whorl on the rim, the other below the rim of the tube, the ovary shortly and abruptly dorsally bulged and the presence of a layer of idioblasts (laticifers?) in the sepals and ovaries. Ctenolophonaceae share with Rhizophoraceae and/or Erythroxylaceae (among other features) sepals with less than three vascular traces, a short androgynophore, an ovary septum thin and severed or completely disintegrating during development, leading to a developmentally secondarily unilocular ovary, a zigzag-shaped micropyle and seeds with an aril. Special features occurring in families of all three groupings studied here are, for example, synsepaly, petals not retarded and thus forming protective organs in floral bud, petals postgenitally fused or hooked together in bud, androecial tube and petals fusing above floral base, androecial corona, apocarpous unifacial styles, nucellus thin and long, early disintegrating (before embryo sac is mature), and nectaries on the androecial tube. Some of these features may be synapomorphies for the entire group, if it forms a supported clade in future molecular studies, or for subgroups thereof. Others may be plesiomorphies, as they also occur in other Malpighiales or also in Celastrales or Oxalidales (COM clade). The occurrence of these features within the COM clade is also discussed

Comparative floral structure and systematics in Chrysobalanaceae s.l. (Chrysobalanaceae, Dichapetalaceae, Euphroniaceae, Trigonaiceae; Malpighiales)

Chrysobalanaceae s.l., one of the few suprafamilial subclades of Malpighiales that is supported by molecular phylogenetic analyses, and containing Chrysobalanaceae, Dichapetalaceae, Euphroniaceae, and Trigoniaceae, was comparatively studied with regard to floral structure. The subclade is well supported by floral structure. Potential synapomorphies for Chrysobalanaceae s.l. are the following shared features: floral cup; flowers obliquely monosymmetric; sepals congenitally united at base; sepals of unequal size (outer two shorter); fertile stamens concentrated on the anterior side of the flower and sometimes united into a strap; staminodes absent in the posteriormost antepetalous position; anthers extremely introrse, with thecae almost in one plane; endothecium continuous over the dorsal side of the connective; dorsal anther pit; gynoecium completely syncarpous up to the stigma; carpel flanks slightly bulged out transversely and thus carpels demarcated from each other by a longitudinal furrow; flowers with dense unicellular, non-lignified hairs, especially on the gynoecium; light-coloured, dense indumentum on young shoots and inflorescences. Potential synapomorphies for Chrysobalanaceae + Euphroniaceae include: spur in floral cup; clawed petals; lignified hairs on petals; nectary without lobes or scales and mostly annular. Potential synapomorphies for Dichapetalaceae + Trigoniaceae include: special mucilage cells in sepals in mesophyll (in addition to epidermis); anthers almost basifixed; gynoecium synascidiate up to lower style; nectary with lobes or scales and semi-annular